This invention relates to detection and cancellation of non-repeatable run-out due to spindle motor cage vibration during a servowrite operation in a disc drive.
Spindle motor vibration is a type of non-repeatable runout (NRRO) that causes motion between the head and media during the rotation of the media by the spindle motor. The NRRO is written into the servo pattern during servowrite operations. Spindle NRRO is composed of low-frequency vibration generated by the ball bearings of the motor, the largest single component being the cage vibration generated by the motor cage. Cage vibration is an extremely low frequency vibration that causes DC track spacing errors and track closure errors during servo writing.
Servowrite operations are performed during manufacture of the disc drive to record servo data onto one or more disc surfaces of the disc drive. The servo writer is affixed to the disc drive and includes a read/write clock head positioned by an actuator arm over the disc to write clock patterns to the disc and to control the data head of the disc drive to write servo data patterns to the disc. In disc drives employing embedded servo patterns, the servo data patterns are written into servo sectors between user fields on each track.
Servo patterns are written on the disc by first writing a clock pattern onto the entire track, or group of tracks, on the disc, including those portions that will become user fields. The clock pattern is usually written over more than one convolution of the disc, over-writing the clock pattern recorded during the prior convolution, to assure that the clock pattern is recorded over the entire track. The end point, where the two recorded convolutions meet, is called the splice point. Next, the track is separated into servo sectors and user data fields, and the disc drive data head writes the servo pattern over the recorded clock patterns in the servo sectors while the clock head reads the clock pattern.
Radial motion of the disc beneath the data head due to spindle motor cage vibration causes the servo writer to write the servo pattern in a spiral, rather than a circle, resulting in a radial discontinuity of the servo pattern at the splice point. As a result, the servo sectors are recorded as segments of that spiral, and not truly on the circular path of the track.
A high magnitude of spindle motor cage vibration during servowriter operations may cause a serious track closure or track squeeze errors, known as track misregistrations. If track misregistrations are severe, the affected tracks are deemed defective during certification of the disc drive, and are tagged for skipping during read and write operations. Consequently, track misregistration diminishes the capacity of the disc media.
Track closure error is evidenced by a position error signal splice during servo pattern write operations that cause servo off-track failures during drive operations. The track squeeze is evidenced by adjacent tracks positioned closer than expected at various locations on the media. This track misregistration is a type of write-to-write track misregistration that generates data cross-talk between adjacent tracks and/or distorts the servo pattern causing defect servo pattern errors.
Experiments on motor cage vibration show that cage vibration is non-synchronous to spindle rotation, but is periodic. The periodic waveform of cage vibration repeats itself over intervals longer than one spindle rotation. Hence, a phase relationship exists between the waveform and spindle rotation. Moreover, the frequency and magnitude of cage vibration is about the same for all motors of same type, regardless of the disc drive system in which they are employed. Hence, the error written in the tracks due to cage vibration has a similar magnitude whether there are track closure errors or not. However, track closure and track squeeze appear to occur randomly due to complicated phase relationships of the cage frequency, spindle rotation and starting phase of the servo write.
U.S. Application Ser. No. 09/483,525 filed Jan. 14, 2000 for xe2x80x9cMethod and Apparatus for Reducing Track Misregistration from Servo Track Writingxe2x80x9d by Xiang Liu, Joseph Liu, Kevin Gomez and Choonkiat Lim and assigned to the same assignee as the present invention, describes placement of a sensor, such as a capacitive or laser interferometer, to measure and extract information concerning cage frequency. Written-in non-repeatable runout due to cage frequency is reduced by starting servo track write operations at selected phases of the cage frequency. However, this technique imposes additional requirements of placement and calibration of the extra sensor. To overcome the problems associated with the extra sensor, it has been proposed to detect the cage frequency with a reference head, or with the clock head of the servowriter, or with the data head of the disc drive. Some of these additional techniques are described in U.S. Application Ser. No. 09/798,801, filed Mar. 2, 2001 for xe2x80x9cDetection and Cancellation of Cage Frequency using the Clock Head in a Servowriterxe2x80x9d by ShuangQuan Min, Ricky Yeo, KianKeong Ooi, Xiong Liu, BengWee Quak and ChiapHeok Ang, assigned to the same assignee as the present invention. These techniques removed significant track closure and track squeeze error during servo write. However, each of these techniques requires modification to the hardware and software of the servowriter to add the function of compensation of cage frequency. The present invention provides a solution to these and other problems associated with cage vibration, and offers other advantages over the prior techniques.
The present invention is directed to a technique by which written-in cage frequency error can be identified and cancelled at the drive certification level.
In one embodiment, written-in runout due to vibration of the cage of a spindle motor of a disc drive is identified. An initial cage frequency of the motor is identified, and the magnitude of written-in successive servo burst closures is read over a plurality of tracks. A maximum servo burst closure is identified from the plurality of read servo bursts, and a magnitude of the cage frequency at a servo sector n0 is calculated based on a difference between the read magnitudes of successive servo burst closures. The phase of the cage frequency is calculated based on the magnitude of the written-in cage frequency at servo sector n0. Preferably, the parameters of the cage frequency profile are stored as values of the cage frequency, maximum servo burst closure and calculated phase. In preferred embodiments, the tracks are segregated into a plurality of zones, and the cage frequency profile is identified for each zone, thereby minimizing the effect of phase drift.
In another embodiment, a track following signal in a rotatable disc drive is compensated for written-in cage frequency runout. A position signal is supplied to an actuator of the disc drive, and the position signal is modified based on the cage frequency profile. In preferred embodiments, the cage frequency profile is read from a memory associated with a controller of the disc drive. The cage frequency profile is added to a signal representing a position disturbance of a read head of the disc drive, and combined with a position command signal for application to the drive controller to operate the actuator.
Yet another embodiment comprises apparatus for compensating a radial position of a head of a disc drive for written-in cage frequency runout. The apparatus includes a controller that provides a position signal to the actuator to control position of the head relative to a selected track on the medium of a disc drive. Cage frequency profile means modifies the position signal based on a cage frequency profile of the disc drive.
In some embodiments of the apparatus, a memory contains a cage frequency profile associated with the disc drive. A summing device, which may be a feed forward controller, combines the cage frequency profile with a signal representing a position disturbance of the head and a position command signal. The drive controller is responsive to the summing device to operate the actuator to control the position of the head relative to the selected track.
These and other features and benefits that characterize the present invention will be apparent upon reading the following detailed description and review of the associated drawings.